RT got back to me, here are the specs of their FD specific cat:

"Part number 903018 has been replaced by part number 141-30038. This converter has 3" diameter inlet and outlet and the body itself is 4" in diameter and 6-1/2" long. The converter flows 617 cfm at a test pressure of 28 inches of water. There is a tube in the pipe that allows an air hose to be connected, but this can be easily capped if you don't want to use it. Capping it will not affect converter operation. Let me know if you need any additional information."

Thanks a lot for the help gracer, they did confirm that its a metal substrate cat (not ceramic).

 

So on my quest for an old man exhaust I've decided on vibrant; the 3" metal substrate converter (listed earlier: http://vibrantperformance.com/catalo...roducts_id=857) and their ultra quiet resonator (http://vibrantperformance.com/catalo...products_id=47).

The other cat option was random technology; their tech dept was great, their 300 cell/sq in converter flows the most and FWIW they even offer a 100cell/sq inch converter (which maxed out their flow tester). But at hundreds of dollars more than the vibrant, I couldn't justify it.

I'm looking to copy the SMB converter/resonator combo (http://shop.jklre.com/products/fd3s-...r-with-muffler) except this will flow more and be MUCH cheaper; with welding I'm hoping to keep this at about $350. I'll hopefully have pics of everything soon.

 

Dude, this is what you want I was searching like mad to find the same information you are after, no one seems to list anything on the effect of catalytic converters. This is what I found after following links off SMB's web site really great information backed up with measurements as well.

All details taken from here > http://www.riceracing.com.au/vbox-ii...ing-tuning.htm



450bhp pressure readings listed here:

 

I used a 3" Magnaflow with V-bands on my last FD. $69 from Summit. Didn't notice a significant loss of power, but don't have back-to-back dyno results to back it up.

http://www.summitracing.com/parts/MPE-94109/

Matt - did you find out something about the Magnaflow option that turned you off?

 

Great datapoints, Rallimike!

Out of curiousity, did you have your wastegate dumped to atmosphere or routed back into the downpipe?

I suspect there would be little HP loss if you used an open dump, since you've eliminated part of the backpressure during the highest load the system will see.

 

"Out of curiousity, did you have your wastegate dumped to atmosphere or routed back into the downpipe?

I suspect there would be little HP loss if you used an open dump, since you've eliminated part of the backpressure during the highest load the system will see."

Routed back to the DP. I'm sure you're right. Problem is, I want a quiet car- oem type quiet.

 

Hey John, nothing specific. I had my mind set on a metal substrate cat due to the better flow characteristics. The vibrant cat flows more than smb (408 cfm) and almost as much as RT (578 to 617 cfm) but is only $115 vs $400+ for RT. Seems like its going work really well.

Exactly - I'm routing my WG back into my DP when the exhaust gets welded; I want a more refined car, making 475 rwhp is less important. Although I hope to keep it over 400 rwhp when this is all done.

 

Thanks for the info. It looks like a nice piece, but from searching around on that link it also looks to be much more expensive than what I'm going to build.

 

It's got little if nothing to do with the waste gate being pumped back into the system, Take the cat out of the system and on the same power you will go from 4psi back pressure to well under 1.5psi on the turbine exit. This then gets multiplied back over the turbine expansion ratio giving big drops in exhaust manifold pressure Brings 17psi back down to 12psi region. The smaller the rear turbine housing you run on your turbo or smaller turbine overall means its more effected by this pressure after the turbine wheel as the smaller units need to operate at larger pressure ratio differentials to flow the same amount of gas which power the compressor to provide the engine with it say 17psi intake boost pressure at 7000rpm. Higher pressure ratio = more exhaust back pressure in the manifold = more pumping losses on the engine = less purging of the clearance volume at top dead center = less power on the same boost air fuel ratio spark timing set up. So on the larger turbine instead of wasting the gas you are using extra flow rate to get the boost you need all while lowering the pressure in the turbine, totally different to how most people think a waste gate works.

When I build my system I am going to run 2 cats of the SMB type listed above in parallel, even then from the figures I have been given this will no where near approach running no cat race mid pipe.

 

Great thread. I have been thinking the exact same thing about my car. Too stinky

 

"When I build my system I am going to run 2 cats of the SMB type listed above in parallel, even then from the figures I have been given this will no where near approach running no cat race mid pipe."

As I previously posted, I've tried this, although I think I did it wrong, in that the Y pipes are too short, creating a lot of turbulence. My next attempt will be to split the exhaust as far front as possible into two 2 1/2" pipes. Then, I want to place a cat and muffler on each pipe, where space permits. I've searched the 'net and emailed both Magnaflow and Borla for guidance on placement of mufflers and cats (front or back placement) and have gotten no help. I'd like to run as clean and as quiet as possible, with as little backpressure as possible . I'm not as smart as many on this board, but it seems to me that Porsche, Corvette, et al. are able to run quiet and clean because their engines divide the exhaust in 2, with cats and mufflers on each pipe. There aren't many cars with 400+ HP that run single exhaust. That, I think, is our problem.

 

The quoted car is running around 440bhp according to SMB when I E-Mailed them about it, and its backed up with multiple different measures, 0-100kmh time, 100kmh-200kmh, measured weight, power and all variables listed along with power, pressures involved.

You are right though there is sfa information around on cars like this.

 

Try contacting random technologies - they have been great getting back to me and are very knowledgeable. I would think two of their 100 cell/sq in cats would be great for the set-up your talking about (the 3" i/o model flows around 670 cfm).
Here's what RT has to say about their 100 cell converters:

"Part number 141-30038 sells for $407.04. The 100-cell version will run approximately $470.
The 100-cell converters are fully functional and will clean up the exhaust almost as efficiently as the 300-cell models. The "problem" is that there's considerably less surface area in a 100-cell converter (than in a 300-cell model) so conversion won't be quite as efficient. The difference in conversion efficiency will be be considerably greater at wide open throttle, than at idle and cruise speeds because of the velocity of the exhaust gasses passing through the converter. The industry term is "resonance time"-- the actual time the exhaust spends passing through the converter. Less resonance time means less time in contact with the converter substrate.
We estimate the flow rate of the 100-cell model to be 670 cfm. Our flow bench can't test beyond 625 cfm, so we have to test at lower pressures and convert."

 

I have seen allot of these melt and fail in very quick time, do any of those makers have guarantees when these are fitted to turbo rotaries? Also the CFM rating seem to be a bit on the high side, do any they list real levels of back pressure @ certain power level like the stuff I found through SMB? That is the only true measure to know what will work in reality.

 

RT said is was a backpressure of "28 inches of water"; not sure how that converts. The vibrant converter did not mention backpressure.

I would assume that cell density, surface area (plane the exhaust passes through) and the length are what affect the flow rate. The RT, SMB and Vibrant converters are 300 cell/sq in (I believe), so the dimensions should give a fair comparison.

 

So that is 1.011psi of pressure caused by the cat

SMB say in their Performance Weaponry 5" body cat like the one I found pictured above it flows the same as a 3.5" straight pipe, yet in the real world testing at 440bhp level there is over 3+psi difference in actual recorded pressure in a car exhaust system.

It seems to me like flow bench figures need to be taken with a grain of salt perhaps

 

From my admittedly weak memory, 400Hp is somewhere in the 800 CFM territory, so two 670 CFM cats would not seem to create much backpressure. I don't spend that much time at full throttle to worry about "resonance time". If they work at idle/cruise, I'm in. Looks like I'm headed towards an expensive exhaust- but, hey, I don't have car payments every month

 

Venting the wastegate to atmosphere (versus plumbing back into the downpipe) reduces pressure in the exhaust manifold. It will allow the motor to make more power.

Plumbing the wastegate back into the downpipe adds exhaust flow post turbo, making the turbine have to work harder to push exhaust out of the system.

Totally understand if people do this for noise reasons - but an open wastegate will allow for more power.

 

Matt,
I'd look into what 'the7wizard' is saying. I believe Carlisi tried a random technologies cat on his car before he went 20B and he had problems with it. On paper it looked great (and the Corvette guys were using them with a lot of success) but I want to say Chris only got a couple k miles out of it before it failed.
Caveat emptor!
Best,
John

 

Damn, I can't believe I found the thread.... and look who the original poster was (!)

https://www.rx7club.com/showthread.p...&highlight=cat

Another thread... with another idea on running them in parallel -
https://www.rx7club.com/showthread.p...at#post5494394

 

Built up a high flow cat a few years ago using a Car Sound (Magnaflow) centerbody 94319.

http://www.car-sound.com/02product/d...niversal=94319

Car ran great, but it definitely felt like midrange power opened up a bit when I went moved from this cat to a resonated MP. Now, I just need to decide if I'm going to move back to the cat and give up some midrange for less stink. Probably not. We'll see since I BARELY drive the car!!

 

Granted having cat can reduce HP but does it also impact spool response? If so how much of a difference is it?

 

I agree - they are basically saying that their cats flow at X CFM at a pressure of 28 inches of water... Who knows how that translates . All I know is that cell density and dimensions should give us everything we need to know for a comparison between two converters .


Thanks for the info John. There's couple of guys who have run them with good results though. Either way, I'm going with the Vibrant, hopefully it lasts, if not it has a 50k warranty .

Nice! That's a whole other story; that SMB cat actually failed on me after only 1k (mentioned that earlier in this thread, the brick came loose); but the turbo I got from A-spec was actually a 4082 instead of a 4088, hence the lack of power (i.e. not all from the cat).


It really shouldn't all that much. Cats don't restrict flow much until higher in the RPM band; usually much past the 3-4.5k needed for a single turbo rotary to spool.

 

I am asking for that information from RR, we will see if he responds

Over email its at least 500rpm slower spool and transient throttle there is always a big pressure differential in the negative against the cat equipped system. So there is very substantial losses involved according to him anyway on the one pictured above, still makes 440bhp though and pressures on engine are favorable.

 

Hmmm... I drove two set-ups on my old fd with and without a cat; one with the twins and one with the GT4082 turbo and didn't notice much (if any) spool difference.

Anyways, got the vibrant parts today (pics attached). I was surprised by how small the converter is, only 8 inches total with maybe 3.5" of catalyst. That may be why the quote a higher CFM than SMB.